This invention relates to a system for transmitting video signals and its control method. When coded digital video signals are transmitted, this system ensures images devoid of image distortion on the reception side even when consecutiveness of the signals is lost.
MPEG2 compression system of the global standard has gained a wide application at present for compressing and transmitting video signals.
The MPEG2 compression system or an equivalent system is based on the premise that input video signals that are compressed are consecutive signals keeping periodicity.
For, compressed image data of one frame are generated in the compression process by using data of a plurality of frames having mutually different time. Unless these frames keep mutual periodicity (both vertical and horizontal periodicities), normal compressed data cannot be acquired.
FIG. 2 of the accompanying drawings shows schematically the outline of input analog video signals. As shown in the drawing, the video signal keeps a predetermined condition as to the relationship between the horizontal sync signal and the vertical sync signal of the video signals in accordance with the standard of the video signals.
The inventor of the present invention has found out that the problem yet to be solved is left when the video signals obtained by outputs of a plurality of surveillance cameras are alternately switched and selected by a switch and the selected video signals from one of the cameras are transmitted after compression and coding to a remote monitor room through a communication line. The video signals from the plurality of surveillance cameras do not always keep a coincident phase and their synchronization is not ensured. Therefore, if a plurality of different video signals are switched and one of the video signals is selectively outputted at the timing that is irrelevant to the vertical and horizontal sync signals of the respective video signals, the vertical sync signal or the horizontal sync signal between different video signals so outputted does not keep predetermined periodicity. Consequently, periodicity of the video signals is distorted.
Consecutiveness of the input video signals is not always ensured when the video signals coded by the MPEG2 compression system are utilized in the surveillance camera system, as will be explained next.
FIG. 3 shows a schematic construction of a multi-camera surveillance system that the inventor of the present invention has examined. Reference numerals 1 and 2 denote video cameras for taking the images under observation. Though ten or more video cameras are employed depending on a system, the example shown in FIG. 3 uses two video cameras.
The output signals 3 and 4 of these cameras are inputted to a video switcher 5 and are switched. One of the outputs is periodically and automatically outputted. The selected signals are inputted as video signals 6 to a transmitter 7. The video signal 6 is converted to a digital composite signal 9 by an A/D converter 8 in the transmitter 7.
The composite signal 9 is inputted to a component conversion portion 10, and is outputted as the video signal that is separated into a luminance component signal and a chromatic component signal, and its sync signal 12. Here, the term “sync signal” represents HR (a signal representing a horizontal effective pixel range), VR (a signal presenting a vertical effective line), ODD (a signal representing a first field), and so forth.
These signals are inputted to a compression portion 13 such as an MPEG2 encoder. The video signal 11 of an effective signal area is compressed on the basis of the sync signal 12 to give a stream 14 as compressed data, that is then transmitted.
This stream 14 is inputted to a receiver 15. After inputted to the receiver 15, the stream 14 is then inputted to a decoder 16. The decoder 16 decodes the stream and outputs a video signal 17 and a sync signal 18.
The video signal 17 and the sync signal 18 are inputted to a composite conversion portion 19 and are outputted as a composite signal 20. The composite signal 20 is converted to an analog video signal 22 by a D/A converter 21. The video signal 22 is inputted to a video monitor 23.
In this way, the image based on the output signals 3 and 4 of the video cameras 1 and 2 switched by the video switcher 5 can be monitored on a video monitor 23.
In such a monitor system, the output signals 3 and 4 of the cameras 1 and 2 are generally asynchronous with each other in most cases.
For this reason, the signal 6 switched by the video switcher 5 becomes an discontinuous signal, that is, a signal having distorted periodicity, at the point of switching.
However, the compressed input video signal in the MPEG2 compression system is based on the premise that the signal keeps periodicity, as described above. Therefore, when the discontinuous signal is inputted to the component conversion portion 10, violation occurs in its internal processing portion with the result that the video signal and the sync signal 12 lose periodicity, too.
A time of dozens of milliseconds is necessary before the component conversion portion 10, to which the discontinuous signal 6 is inputted, recovers to be normal operation and outputs a regular sync signal 12.
On the other hand, the operation of the compression portion 13, to which the discontinuous video signal and the sync signal 12 are inputted, gets distorted, too, and generates error data in the resulting stream 14.
Therefore, the decoder 16 of the receiver 15 generates the noise due to the compression-coded data error, and the image collapses or gets distorted on the video monitor 23.
FIG. 4 shows schematically the mode of this trouble. The drawing shows the case where the signals 3 and 4 are inputted asynchronously with their phases dephasing from each other, and the output of the video switcher 5 is changed over from the signal 3 to the signal 4 at a certain point of time.
In the video signal 6 so switched and selected, a discontinuous portion develops at the switching point of the signal as shown in FIG. 4.
This discontinuous point triggers and invites distortion of periodicity in the sync signal 12 as the output of the component conversion portion 10 for a period of dozens of milliseconds. The consecutive video signal 11 based on the signal 4 and the sync signal 12 are thereafter outputted.
As a result, collapse and distortion of the image occur eventually in the video monitor 23.
To solve this problem, it has been necessary to use a video switcher having a frame synchronization function for the video switcher 5.
This frame synchronization function can synchronize a plurality of asynchronous input signals by signal processing and can output one of them selectively. When this synchronization switcher is used, the occurrence of the collapse and distortion of the image can be prevented. The synchronization function once writes a plurality of input video signals into frame memories and reads any one of them out from the frame memories by a common read signal.
However, because the video switcher equipped with the frame synchronization function is very expensive, the increase of the cost becomes a serious problem particularly in a surveillance system that handle the signals of a large number of cameras.